Well, the zerker can't really be called "OP" Per say. Only the horn skill can be called OP because, well, it is... sorta. The regular attack, along with all of the other skills can be dodged pretty easily, and berserk just makes a bigger target.
Now, back to the horn. It's one of those things that if you get hit by it, you're screwed. The stun is so long that you can actually go and ram someone else, run back to the person you stunned, and hit them. But, on the other hand, you can just get out of the way and beat them up when they aren't using the horn.
Boxer, now the BOXER is OP. The guy can shove out more combos faster than some characters can use their counter. (Poor shadow assassin.) All you have to do with this guy is mash the D key, and the knockback and advance from each hit is just... insane, even when blocked.
It wouldn't be so bad if there was a slightly longer delay between combos.
There's My $.02